need help on a digital100amp volt meter

[rambo99]

My son buy a DC 4.5-30V 0-100A Dual LED Digital Volt Ammeter Voltage AMP Power from ebay and he connect to a 12v power supply this power supply is 85amp which power a car amplifier which work very good he is trying to connect this digital meter so he can check to see how much amp is dropping down from the power supply but it seem the meter don't work that way I connect the meter to a battery and a 12v motor and the meter work ok it shows the volts and the amp is any way to set this meter so the volts and amp shows or this meter can only work with load

 

[Harald Kapp]

or this meter can only work with load

An ampere meter can only work with a load. It is designed to show the current flowing through the load. If there's no load, there's no current.
You connect the ammeter in series with the load, the voltmeter parallel to the load.